The invention pertains to a device for testing the shear strength of soils. In particular, the invention pertains to a penetrometer useful for both high strength granular road and airfield pavement layers and relatively weak underlying subgrade type soils.
One of the most important soil properties is shearing resistance or shear strength which is related to the ability of the soil to withstand a load. This is especially important in relation to the supporting strength or bearing capacity of a soil used as a base or subgrade beneath a road, runway or other structure.
For most military applications, the well known California Bearing Ratio (CBR) value of a soil is used as a measure of shear strength. The CBR is determined by a penetration shear test and is used with empirical curves for designing and evaluating unsurfaced, aggregate surfaced and flexible pavements for military roads and airfields. The CBR test is usually performed on laboratory-compacted test specimens. When used on-sight for pavement evaluations, destructive test pits are dug to determine pavement layer thickness. Field or on-sight CBR tests are conducted on the base course, the sub-base, and sub-grade materials. On-sight CBR tests are time consuming to run and are generally not practical for use in a theater of operations.
For unsurfaced roads and airfields, a known airfield cone penetrometer is used to determine index of soil strengths (airfield index). The airfield penetrometer consists of a 30.degree. cone with a 0.2 square inch base area. The force required to penetrate to various depths in the soil is measured by a spring, and the airfield index is read directly from the penetrometer. The airfield cone penetrometer has a range of 0-15 which corresponds to a CBR value of approximately 0-18. While the device is compact, sturdy and simple enough to be used by military personnel experienced in soil strength determination, a major drawback of the airfield cone penetrometer is that it will not penetrate various types of crusts or thin base course layers that may overlie softer layers. Relying only on the surface airfield index test could, under some conditions, result in the loss of a vehicle or an aircraft.
Another device for measuring shear strength is a single mass dynamic cone penetrometer. The device has only a single heavy mass designed to penetrate strong soils up to a CBR of 100. It cannot, however, measure the strength of very weak soil layers for example, CBR less than 1. Also, the single heavy mass penetrometer is too heavy to accurately measure the strength of clay soils having a CBR of 10 or less. The single mass device may also penetrate thin crust layers in less than one hammer drop, and actually measure the average strength of the weak and crust layers instead of the actual strength of each layer.
There are known devices which use a single mass and conical penetrometer tips of various kinds. See for example, Chua et al., Transportation Research Record 1192, 1988, which shows a single mass device with an attached scale. Livneh et al, Proc., 6th International Conference on Structural Design of Asphalt Pavements, Jul. 1987, shows a single weight penetrometer with a 30.degree. cone. Van Vuuren, The Rhodesian Engineer, vol. 7, no. 5, pp 852-854, shows a single weight device (10 Kg) with a 460 mm drop height. None of the references show devices which conveniently produce desirable results for both the weak and strong soil conditions.